1. Field of the Invention
The present invention relates to a push switch apparatus, and more particularly, to a push switch apparatus which is capable of obtaining an overstroke after the switch is switched on.
2. Description of the Related Art
A structure of a conventional push switch apparatus has been disclosed (for example, see Japanese Unexamined Utility Model Registration Application Publication No. 2-92620). The push switch apparatus according to the related art has an element switch, in which a fixed contact is exposed from an inner bottom surface of a housing and a movable contact having an inversion spring contacting/separating with/from the fixed contact is housed inside the housing, a key top that ascends and descends along the element switch, and a coil spring that is interposed between the key top and the element switch.
Hereinafter, the structure of the push switch apparatus according to the related art will be described with reference to the drawing.
FIG. 9 is a cross-sectional view of the push switch apparatus according to the related art.
In FIG. 9, a boss 11a is provided at a ceiling portion of a cap-shaped key top 11, and an upper end of a coil spring 12 is press-fitted into the boss 11a. Further, engaging protrusions 11b are provided at two places of a lower end of the key top 11. In an element switch 13, two metal pieces separated from each other are embedded in a bottom portion of a boxlike housing 14 such that portions of both metal pieces are exposed from an inner bottom surface of the housing 14 as fixed contacts 15 and 16. An inversion spring 17 is provided on the fixed contacts 15. The inversion spring 17 functions as a movable contact separated from the fixed contacts 16, and a dustproof sheet 18 is attached so as to be positioned to cover these contacts 15, 16, and 17. A lower end of the coil spring 12 is pressed against a central portion of the inversion spring 17 through the sheet 18.
In addition, an outer wall surface of the housing 14 functions as a slid surface of the key top 11, and removal preventing protrusions 14a are provided at two places of the outer wall surface so as to be engaged with the engaging protrusions 11b. Terminals that extend from the fixed contacts 15 and 16 protrude in a side direction of the housing 14 from the lower end of the housing 14, and a positioning protrusion 14b stands upright at the bottom surface of the housing 14.
The above-described key switch is surface-mounted on a printed board 20 by attaching the respective terminal 19 to land portions (not shown) via solders 21 after the positioning protrusion 14b of the housing 14 is fitted into a positioning hole 20a of the printed board 20.
In this key switch, the key top 11 can freely ascend and descend along the outer wall surface of the housing 14. If an operator presses the key top 11 by a predetermined amount, the lower end of the coil spring 12 causes the inversion spring 17 to be inverted, such that both the fixed contacts 15 and 16 are conducted to each other and the key switch is switched on. If the operator further presses the key top 11, the coil spring 12 is bent, thus an overstroke is obtained.
At this time, the operator feels a click due to the inversion movement of the inversion spring 17 through his finger. If the press is released from the key top 11, which is switched on, the inversion spring 17 is separated from the fixed contact 16 with its elasticity, and thus the key switch is switched off. Sequentially, the key top 11 ascends with resilience of the coil spring 12 and stops at the time when the engaging protrusion 11b is engaged with the removal preventing protrusion 14a. 
However, in the structure of the conventional push switch apparatus, an initial spring pressure of the coil spring 12 is set smaller than the operation force of the inversion spring 17, and the inversion is made when the pressure against the key top 11 exceeds the operation force of the inversion spring 17. Accordingly, it is difficult to achieve the configuration in which the switch is pressed right at the beginning of the stroke, and then an overstroke is obtained. As a result, there is a problem in that the degree of freedom cannot be given in an operation mode.
In addition, if the initial pressure of the coil spring 12 exceeds the operation force of the inversion spring 17, the switch is unnecessarily switched on, even when the key top 11 is not pressed. Accordingly, the switch may be accidentally switched on by the variation in spring characteristic of the coil spring 12. Further, stroke may be varied until the switch is switched on.